# std::function<R(Args...)>::operator=

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< cpp‎ | utility‎ | functional‎ | function

function& operator=( const function& other ); | (1) | (since C++11)
function& operator=( function&& other ); | (2) | (since C++11)
function& operator=( std::nullptr_t ) noexcept; | (3) | (since C++11)
template< class F > function& operator=( F&& f ); | (4) | (since C++11)
template< class F > function& operator=( std::reference_wrapper<F> f ) noexcept; | (5) | (since C++11)

Assigns a new target to `std::function`.

1) Assigns a copy of target of other, as if by executing function(other).swap(*this);
2) Moves the target of other to *this. other is in a valid state with an unspecified value.
3) Drops the current target. *this is empty after the call.
4) Sets the target of *this to the callable f, as if by executing function(std::forward<F>(f)).swap(*this);. This operator does not participate in overload resolution unless f is Callable for argument types `Args...` and return type `R`.
5) Sets the target of *this to a copy of f, as if by executing function(f).swap(*this);

### Parameters

other | - | another `std::function` object to copy the target of
f | - | a callable to initialize the target with
Type requirements
- `F` must meet the requirements of Callable.

### Return value

*this

### Notes

Even before allocator support was removed from `std::function` in C++17, these assignment operators use the default allocator rather than the allocator of *this or the allocator of other (see LWG issue 2386).

### Example

Run this code

    #include <cassert>
    #include <functional>
    #include <utility>

    int inc(int n) { return n + 1; }

    int main()
    {
        std::function<int(int)> f1;
        std::function<int(int)> f2(inc);
        assert(f1 == nullptr and f2 != nullptr);

        f1 = f2; // overload (1)
        assert(f1 != nullptr and f1(1) == 2);

        f1 = std::move(f2); // overload (2)
        assert(f1 != nullptr and f1(1) == 2);
        // f2 is in valid but unspecified state

        f1 = nullptr; // overload (3)
        assert(f1 == nullptr);

        f1 = inc; // overload (4)
        assert(f1 != nullptr and f1(1) == 2);

        f1 = [](int n) { return n + n; }; // overload (4)
        assert(f1 != nullptr and f1(2) == 4);

        std::reference_wrapper<int(int)> ref1 = std::ref(inc);
        f1 = ref1; // overload (5)
        assert(f1 != nullptr and f1(1) == 2);
    }

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2132 | C++11 | the overload (4) taking a Callable object might be ambiguous | constrained
LWG 2401 | C++11 | assignment operator (3) from `std::nullptr_t` not required to be noexcept | required

### See also

operator= | replaces or destroys the target (public member function of `std::move_only_function`)
assign (removed in C++17) | assigns a new target (public member function)

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